2013

Petrogenetic Evidence from Clay Howells (Ontario) for Auto-Oxidation, Magnetite Fractionation, and LREE-Nb Saturation in (Ferro)carbonatite: implications for the origin of Bayan Obo (China)?

David Richard Lentz

University of New Brunswick, Department of Earth Sciences

The Mesoproterozoic Clay Howells LREE & Nb deposit is the youngest and one of the richest carbonatite-related deposits in the Kapuskasing Magmatic Province. It is situated situated within Clay Howells Alkaline Complex (CHAC) that has prominent ring-like structures. The carbonatitic part of the system is slightly discordant to SE outer ring features that are evident from the airborne vertical gradient magnetic maps of the entire CHAC. The deposit itself is characterized by cumulate-like magnetite (20 to >90%) layers with interstitial calcite in two zones dipping moderately towards the magmatic center of the main CHAC. The body itself as drilled to date is > 800 m long, with widths up to 100m based on drilling, although exploration is not complete on this body. Macroscopic features, gamma-ray spectrometry results, and assay data show that the LREE, Nb, and U, and Th abundances are related to the most magnetite-rich cumulates. The saturation of LREE and Nb phases and magnetite indicates that the saturation of magnetite in the intruding carbonatite triggers saturation of these key phases, i.e., there is a complex solution relationship  of Fe-P-La-Nb-Th in the melt. The differential exsolution of H₂ (autooxidation) from a hydrous ferrocarbonatite in a subvolcanic environment could be responsible for the partial oxidation of Fe triggering magnetite saturation. The proportion of hydrogen gas, CO & CO₂, and water would be high, but rapidly decreasing during ascent and emplacement. To form these mineralized magnetite cumulates,  the proportion of ferrocarbonatite magma was much larger than observed here. Although there are many other LREE or Nb mineralized carbonatite bodies, few are enriched in both LREE and Nb (i.e., Bayan Obo, Palabora). When magnetite saturates in a very low viscosity carbonatite, magnetite very quickly settles, with very minimal evidence of its origin as a sideritic component in an extremely fractionated ferrocarbonatite.